The incorporation of elaidic acid into Ehrlich ascites tumor cells (EATC) upon feeding the host an elaidic acid-rich diet has been investigated. The EATC lipids contained only one-half the concentration of elaidic acid found in the lipids of either the host livers or of livers from normal mice. On the other hand, elaidic acid incorporation into tumor cells was close to that of ascites fluid. This incorporation was mainly into phospholipids; the highest into choline phospholipids and ethanolamine phospholipids. EATC phospholipids had reduced poly-unsaturated fatty acids as compared with oleic acid-grown cells. Tumor development was accompanied by an increase in elaidic acid of the host's liver. Only in EATC, elaidic acid incorporation in tissues studied resulted in a reduction in the amount of all major lipids in the tumor. The development of the tumor did result in decreases in triacylglycerols and esterified cholesterol, and increase in phospholipids and free cholesterol in the livers of host animals.